Rembering the Land
Purim is not celebrated on the same day everywhere. In walled cities, we are told in Megillat Esther, the celebration is on the 15th of Adar, while in cities without walls it is on the 14th.
The reason for this is that in the unwalled cities, the Jews overcame their enemies on the 13th of Adar and celebrated on the 14th, while in the walled capital of Shushan the battle still raged on the 14th, and the Jews there could not celebrate their victory until the 15th. Therefore, all walled cities celebrate Purim on the 15th because of their similarity to Shushan.
The designation "walled city" does not depend on a city’s present situation, but rather on whether it had a wall at the time Joshua led the Jewish nation in the conquest of Eretz Yisrael. But why is Joshua’s time the criterion for the designation "walled city?" Wouldn’t it have been more logical to make this determination based on the time of the Purim miracle?
The answer is found in the Jerusalem Talmud where Rabbi Yehoshua ben Levi declares that this was done in order to accord honor to Eretz Yisrael which lay desolate at the time of the Persian miracle.
Rabbi Nissan ben Reuven (Ran) explains this as follows: At the time of the Purim miracle there were hardly any cities in Eretz Yisrael with their walls still intact. Had the determination of "walled city" been made according to the situation at that time, almost all cities in Eretz Yisrael would have the status of unwalled cities. To avoid this disgrace, it was decided to base the status "walled city" on the situation of the city at the time of Joshua. This made many more cities in Eretz Yisrael eligible for this distinction.
Rabbi Yosef Karo (Beis Yosef) has a different approach. Our Sages wanted some memory of Eretz Yisrael in the celebration of this miracle which took place in a foreign land. In the spirit of "zecher lemikdash" — those laws and customs we follow to recall the Beit Hamikdash — the Sages linked the determination of "walled city" to Eretz Yisrael so that the Jews living abroad would not forget their holy land.